Hybrid Resources

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Objective

This is a response to #24591 and #24592.

In short, during the resources-as-components proces it was decided that trait Resource : Component instead of using a wrapper component like ResourceData<R: Resource>(R). This caused one major regression:

#[derive(Component, Resource)] // error -> can't derive Component twice!
struct Foo;

Double derives were no longer possible. This is occasionally useful. One particular example would be global settings with local overrides. You would set a global default which you could locally override for a specific entity - all within the same type. Because this was now disallowed, you had to split your struct into two:

#[derive(Component)]
struct LocalFoo;

#[derive(Resource)]
struct GlobalFoo;

This is verbose and generally not ideal.

Solution

Introduce HybridResource: Types that implement this trait function both as a Resource and a Component, roughly mimicking the old #[derive(Resource, Component)] behaviour.

Problems

• I drew this up rather quickly, so this is only a draft. It's more meant as a proof of concept. The name is bikesheddable.
• The hooks I currently have can very easily be cleaned up with components-as-entities
• Currently the hooks can't make a distinction between world.spawn() and world.insert_resource() so if you do world.spawn() before doing world.insert_resource() it will assume that the world.spawn()ed value is the resource, not the component. A real solution might just have to bring back the wrapper type.